We want to balance the rights of current and future generations, and this gives a public policy question: how long after your death should you still be able to control the use of what was yours? Since 1682 the answer to "how long" has been the relatively complicated Rule Against Perpetuities, setting the limit as:

twenty-one years after the death of some life in being at the creation of the interest.

This is a very complicated way of defining a time period, but at least for most of history it's been relatively limited. Making things confusing, however, is that an unborn child is considered to be a "life in being" for the purposes of this rule. While no one has tried this in court, it's possible that this includes frozen embryos. [1] In which case you could make a perpetual trust by specifying a cryogenically preserved embryo as the "measuring life" for the purposes of the trust, and including in the trust the stipulation that the embryo remain frozen.

I kind of hope someone does this so I can read the court's opinion, but policywise we should probably just scrap the traditional rule and switch to a simple fixed limit of N years.

[1] Extensive discussion in Gametes, Embryos and the Life in Being: The Impact of Reproductive Technology on the Rule Against Perpetuities (McCrimmon, 2000).